What happened when you spoke to the employee?  Does it happen suddenly without warning or are there pre-emotive symptoms?  What task are they performing.  

https://www.nhs.uk/conditions/fainting/

Do any of the precursors apply - in particular being thirsty or having skipped meals? It is not uncommon for parents to forego food so their children can eat.

We get this with students a lot: they skip breakfast and are not used to standing around for a long time. Some wear too many layers of clothing.

Find out what could be causing this; is it something  in the home eg skipping meals ;is it the job itself are they simply working too hard   with no breaks. What did the medic say and which medics. General practitioners and the like,  will just look at obvious signs of diseases while an occupational health specialist  will look at how the job might effect the person and identify factors which are not signs disease per se but will explain the fainting spells.

Agree with whats been said above, have a chat with employee and see if they are willing to explain why (but they dont have to) they may be working more than 1 job, be subjected to abuse at home who knows, a good employer would try and descreatly find out a little more- make a OH referal if you are worried. But form a H&S point of view are then any potential causes of fainting in the workplace (unlikly in most) and what are the potential conesequences? If this person works in an office then i think the risks are minimal - if they are drioving a FLT i would have real concerns. (just an example as i know as i know we emply different rules to fittness to work in different situations.)

Thanks for taking themtime to reply and all the good advice. You have given me more questions to answer  - This will be a difficult one to deal with as each time the have fainted they claim not to remember anything that happened to them.